Auto merge of #41905 - frewsxcv:rollup, r=frewsxcv

Rollup of 5 pull requests

- Successful merges: #41192, #41724, #41873, #41877, #41889
- Failed merges:

3 files changed, 86 insertions, 24 deletions

diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs
index e2832873e2e..c872a8e5261 100644
--- a/src/libstd/io/mod.rs
+++ b/src/libstd/io/mod.rs
@@ -287,9 +287,11 @@ pub use self::error::{Result, Error, ErrorKind};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::util::{copy, sink, Sink, empty, Empty, repeat, Repeat};
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use self::stdio::{stdin, stdout, stderr, _print, Stdin, Stdout, Stderr};
+pub use self::stdio::{stdin, stdout, stderr, Stdin, Stdout, Stderr};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::stdio::{StdoutLock, StderrLock, StdinLock};
+#[unstable(feature = "print_internals", issue = "0")]
+pub use self::stdio::{_print, _eprint};
 #[unstable(feature = "libstd_io_internals", issue = "0")]
 #[doc(no_inline, hidden)]
 pub use self::stdio::{set_panic, set_print};
diff --git a/src/libstd/io/stdio.rs b/src/libstd/io/stdio.rs
index 38ad23e14b3..a8b0bf0071a 100644
--- a/src/libstd/io/stdio.rs
+++ b/src/libstd/io/stdio.rs
@@ -17,7 +17,7 @@ use io::{self, BufReader, LineWriter};
 use sync::{Arc, Mutex, MutexGuard};
 use sys::stdio;
 use sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
-use thread::LocalKeyState;
+use thread::{LocalKey, LocalKeyState};
 
 /// Stdout used by print! and println! macros
 thread_local! {
@@ -659,41 +659,56 @@ pub fn set_print(sink: Option<Box<Write + Send>>) -> Option<Box<Write + Send>> {
     })
 }
 
-#[unstable(feature = "print",
-           reason = "implementation detail which may disappear or be replaced at any time",
-           issue = "0")]
-#[doc(hidden)]
-pub fn _print(args: fmt::Arguments) {
-    // As an implementation of the `println!` macro, we want to try our best to
-    // not panic wherever possible and get the output somewhere. There are
-    // currently two possible vectors for panics we take care of here:
-    //
-    // 1. If the TLS key for the local stdout has been destroyed, accessing it
-    //    would cause a panic. Note that we just lump in the uninitialized case
-    //    here for convenience, we're not trying to avoid a panic.
-    // 2. If the local stdout is currently in use (e.g. we're in the middle of
-    //    already printing) then accessing again would cause a panic.
-    //
-    // If, however, the actual I/O causes an error, we do indeed panic.
-    let result = match LOCAL_STDOUT.state() {
+/// Write `args` to output stream `local_s` if possible, `global_s`
+/// otherwise. `label` identifies the stream in a panic message.
+///
+/// This function is used to print error messages, so it takes extra
+/// care to avoid causing a panic when `local_stream` is unusable.
+/// For instance, if the TLS key for the local stream is uninitialized
+/// or already destroyed, or if the local stream is locked by another
+/// thread, it will just fall back to the global stream.
+///
+/// However, if the actual I/O causes an error, this function does panic.
+fn print_to<T>(args: fmt::Arguments,
+               local_s: &'static LocalKey<RefCell<Option<Box<Write+Send>>>>,
+               global_s: fn() -> T,
+               label: &str) where T: Write {
+    let result = match local_s.state() {
         LocalKeyState::Uninitialized |
-        LocalKeyState::Destroyed => stdout().write_fmt(args),
+        LocalKeyState::Destroyed => global_s().write_fmt(args),
         LocalKeyState::Valid => {
-            LOCAL_STDOUT.with(|s| {
+            local_s.with(|s| {
                 if let Ok(mut borrowed) = s.try_borrow_mut() {
                     if let Some(w) = borrowed.as_mut() {
                         return w.write_fmt(args);
                     }
                 }
-                stdout().write_fmt(args)
+                global_s().write_fmt(args)
             })
         }
     };
     if let Err(e) = result {
-        panic!("failed printing to stdout: {}", e);
+        panic!("failed printing to {}: {}", label, e);
     }
 }
 
+#[unstable(feature = "print_internals",
+           reason = "implementation detail which may disappear or be replaced at any time",
+           issue = "0")]
+#[doc(hidden)]
+pub fn _print(args: fmt::Arguments) {
+    print_to(args, &LOCAL_STDOUT, stdout, "stdout");
+}
+
+#[unstable(feature = "print_internals",
+           reason = "implementation detail which may disappear or be replaced at any time",
+           issue = "0")]
+#[doc(hidden)]
+pub fn _eprint(args: fmt::Arguments) {
+    use panicking::LOCAL_STDERR;
+    print_to(args, &LOCAL_STDERR, stderr, "stderr");
+}
+
 #[cfg(test)]
 mod tests {
     use thread;
diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index a1f092621cb..ef78ea6dfe8 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -68,6 +68,9 @@ macro_rules! panic {
 /// necessary to use `io::stdout().flush()` to ensure the output is emitted
 /// immediately.
 ///
+/// Use `print!` only for the primary output of your program.  Use
+/// `eprint!` instead to print error and progress messages.
+///
 /// # Panics
 ///
 /// Panics if writing to `io::stdout()` fails.
@@ -105,9 +108,12 @@ macro_rules! print {
 /// Use the `format!` syntax to write data to the standard output.
 /// See `std::fmt` for more information.
 ///
+/// Use `println!` only for the primary output of your program.  Use
+/// `eprintln!` instead to print error and progress messages.
+///
 /// # Panics
 ///
-/// Panics if writing to `io::stdout()` fails.
+/// Panics if writing to `io::stdout` fails.
 ///
 /// # Examples
 ///
@@ -124,6 +130,45 @@ macro_rules! println {
     ($fmt:expr, $($arg:tt)*) => (print!(concat!($fmt, "\n"), $($arg)*));
 }
 
+/// Macro for printing to the standard error.
+///
+/// Equivalent to the `print!` macro, except that output goes to
+/// `io::stderr` instead of `io::stdout`.  See `print!` for
+/// example usage.
+///
+/// Use `eprint!` only for error and progress messages.  Use `print!`
+/// instead for the primary output of your program.
+///
+/// # Panics
+///
+/// Panics if writing to `io::stderr` fails.
+#[macro_export]
+#[stable(feature = "eprint", since="1.18.0")]
+#[allow_internal_unstable]
+macro_rules! eprint {
+    ($($arg:tt)*) => ($crate::io::_eprint(format_args!($($arg)*)));
+}
+
+/// Macro for printing to the standard error, with a newline.
+///
+/// Equivalent to the `println!` macro, except that output goes to
+/// `io::stderr` instead of `io::stdout`.  See `println!` for
+/// example usage.
+///
+/// Use `eprintln!` only for error and progress messages.  Use `println!`
+/// instead for the primary output of your program.
+///
+/// # Panics
+///
+/// Panics if writing to `io::stderr` fails.
+#[macro_export]
+#[stable(feature = "eprint", since="1.18.0")]
+macro_rules! eprintln {
+    () => (eprint!("\n"));
+    ($fmt:expr) => (eprint!(concat!($fmt, "\n")));
+    ($fmt:expr, $($arg:tt)*) => (eprint!(concat!($fmt, "\n"), $($arg)*));
+}
+
 /// A macro to select an event from a number of receivers.
 ///
 /// This macro is used to wait for the first event to occur on a number of